Wall Wart Tube Power

[jackinnj]

I didn't have time to get a transformer for a project which is a Christmas gift -- here's what I hooked up -- the wallwarts are beefy 20 watt types used in Zip Drives -- as external drives are all USB now they are collected in a box in the shop -- if you are going to test out these small transformers use a conventional light bulb in series to "current limit".

[EC8010]

Please tell me that you aren't really using a half-wave rectifier for the heater supply! Half-wave rectification puts a very nasty load on a transformer because it causes it to pass DC - which easily saturates the core.

[jackinnj]

it's a full wave rectifier -- just too lazy to draw in all the diodes -- the left hand wall wart was left "intact" with its diodes and capacitor --

[dhaen]

I'm surprised the 5V 'wart TX doesn't saturate...

[DigitalJunkie]

the left hand wall wart was left "intact" with its diodes and capacitor --

So it's output is DC?

[Brian Donaldson]

So your putting 9v into the 2.5v worth of coil. Unless your getting out 400v+, your asking for drama.

You know better than that.

Oh wait, I see, it was 2 diode half bridge. Your only 100% above rated voltage.

[jackinnj]

with 117 VAC in I am getting 353VDC out of the voltage doubler and 6.299V to the filament.

note that the adjust leg of the LM317 is NOT connected to ground -- it floats.

[EC8010]

I'll bet you really wish you hadn't started this thread...

Have you looked at the voltage waveform going into the HT voltage doubler? If it hasn't got flat tops then your transformer core isn't saturating.

[planet10]

Do i smell smoke?

dave

[jackinnj]

no smoke yet -- there seems to be about a diode drop's worth of DC on the transformer coupling -- so there is some saturation going on -- this under load -- the temperature rise (can only measure the exterior of the transformer with a laboratory thermometer) is a few degrees C after 1/2 hr.



and the waveform (not pretty) between the two transformers (you can see the diode ringing.)